Heat exchanger



NOV. I7,

Filed Sept. 15, 1947 M. FRENKEL 2,659,392

HEAT EXCHANGER 4 Sheets-Sheet l M. FRENKEL- HEAT EXCHANGER Nov. 17, 19534 Sheets-Sheet 2 Filed Sept. 15, 1947 INVENTOR Nov. 17, 1953 M. FRENKEL2,659,392

HEAT EXCHANGER Filed Sept. 15, 1947 4 Sheets-Sheet 3 INVENTOR M. FRENKELHEAT EXCHANGER Nov. 17, 1953 4 Sheets-Sheet 4 Filed Sept. 15, 1947INVENTOR FIG. 10.

Patented Nov. 17, 1953 UNITED STATES PATENT OFFICE HEAT EXCHANGER MeyerFrenkel, London, England Application September 15, 1947, Serial No.774,151

7 Claims. (01. 138-38) This invention relates to apparatus for effectingheat exchange between two fluids, or between a fluid and a heating orcooling element and especially to apparatus such as radiators forinternal combustion engines, oil coolers for internal combustion enginesand other purposes, chemical heat exchangers, condensers and evaporatersfor use in refrigerating plant and for other purposes.

More particularly, the, invention relates to heat exchangers in whichsecondary walls, i. e. walls connected with the primary heattransferring walls separating the media partaking in the heat exchange,and immersed in one of the fluids taking part in the heat exchange, areprovided for the purpose of transferring heat by conduction between theprimary heat transferring walls and layers of the fluid coming into heatexchange contact therewith.

Now before coming to the invention itself, consider the heat exchangefor a fluid flowing between two plates, whether primary heattransferring walls or secondary heat transferring walls, when these arenot very close together.

Even with large velocities of flow producing strongly turbulent flow,the vortices in a fluid develop mainly in a thin layer adjacent thewall, and rapidly fall off towards the centre of the flow-cross-section.Hence the mixing and thus heat exchange of particles mainly takes placeadjacent the heat transferring walls, there being hardly any mixing inthe middle layers, so that, particularly for a bad conductor, the fasterflowing middle layers of the flow hardly take any part in theheat-exchange.

It will be seen that even in the turbulent layers near the wall thereis:

1. Mixing of particles which have taken part in the heat exchange amongthemselves,

2. Mixing of particles which have not yet taken part in the heatexchange among themselves, all of which is useless for the heatexchange, and

3. Only to a smaller extent mixing of particles which have taken part inthe heat exchange with particles which. have not yet taken part in theheat exchange, which is the only kind of mixing useful for the heatexchange.

Hence for a required mean temperature of the fluid emerging from thepassage, the outer layers have experienced a much greater temperaturechange than is required, so that the temperature difference between theouter layers and the walls will have fallen very quickly in the flowdirection and with it the rate of heat transfer per unit area of heattransferring wall per unit' volume of fluid flowing therealong, so thatthe mean rate of heat transfer per unit area of heattransferring walltaken over the length of the passage is low. Moreover, although thefluid will leave the passage with a required mean temperature, differentlayers will leave the passage with widely differing temperatures.

Thus, in order to achieve in such passages, a transfer of a certainquantity of heat, very large surface areas of heat transferring wall arerequired.

If now secondary walls extending the whole length of the passage andspaced relatively far apart are provided, then with turbulent flow thesame phenomena will occur. Moreover, stagnant boundary layers form onthe secondary walls as well as on the main heat transferring walls andprovide layers of low heat conductivity between the flowing fluid andthe metal, the thickness of the stagnant boundary layers being afunction of the temperature and, in the case of narrow, elongatedcross-sections, being inversely proportional to the distance between theheat-transferring walls. These stagnant boundary layers considerablyreduce the rate of heat transfer per unit area of heat transferringwall, and further restrict the effective crosssectional area of flow ofthe passage.

One object of the present invention is to provide heat exchangers inwhich the surface area and weight of the secondary walls and the primaryheat transferring Walls is considerably reduced as compared withconventional heat exchangers, for the same performance.

A further object of the invention is to ensure that each layer of thefluid stream, irrespective of its thickness for effective heat exchange,emerges from the heat exchanger passage with substantially the sametemperature.

Still a further object of the invention is to reduce pressure losses forthe fluids flowing through and taking part in the heat exchange.

Still a further object of the invention is to hinder the formation ofstagnant boundary 1ayers on the secondary walls.

Still a further object of the invention is to raise to a substantialextent the rate of heat transfer per unit area of heat transferring wallas compared with conventional constructions.

Further objects and advantages will become apparent from the followingdescription.

With the foregoing objects in view, the Present invention provides:

A heat exchanger comprising at least one primary heat transferring wallforming, at least in part, a channel for a heat exchange fluid flowingtherealong, and a series of secondary heat transferring walls, of whicheach extends alon said channel and has a fraction of the length of saidprimary heat transferring wall along said channel, the said secondarywalls of said series being in a staggered arrangement relative to oneanother with, for at least two consecutive secondary walls of: saidseries, a gap along said channel between the trailing'edge of the oneand the leading edge of the consecutive secondary wall,

and with at least said two secondary walls of said series out ofalignment along the flow-lines of said fluid with any secondary wall insaid channel, and at least one of said two secondary walls having twoopposite edges connected. to a primary heat transferring wall, the saidedges extending along said channel.

In such a staggered arrangement of secondary walls, those out ofalignment along the flow lines of said fluid may be displaced parallelrelative to one another, or displaced rotationally relative to oneanother, or displaced through a combination of parallel and rotarydisplacement.

Accordingly, the orthogonal projections of trailing and leading edgesof. adjacent secondary walls onto a flow-cross-section of said fluid inthe gapbetween said secondary walls may or may not be parallel, and mayor may not intersect, and they may, of course, be of similar of ordifferent shapes.

It will further be understoodthat the secondary walls may be plane ormay be curved, e. g. cormgated, or there may be plane and curved wallsin oneseries.

In preferred embodiment of the invention, the secondary walls of aplurality of serie are arranged. in step-wise staggered progressionrelative to one another, with a gap along said passage between thetrailing edge of eachsecondary wall and the leadingedge of theconsecutive secondary wall, and with each secondary wall of each seriesout of alignment with all other secondary walls in the passage, and eachsecondary wall of a. series having twoopposite edges connected with saidprimary wall, the said edges extending along said-passage, and the saidcontacts with said primary wall being arranged in an evenly distributivepattern thereon.

The invention will now be described by way of example and in some detailwith reference to the accompanying drawing, in which:

Fig. 1 is a plan section of, part of an oil-cooler for an air-craft;

Fig. 2 isa section inv elevation through one o of: this part, along theline 11-11 of Fig. 1;

Fig. 3 is another. section. in elevation along a passage of this part,along the line III--III of .1:

Fig. 4 is-apart isometric view of an assembled oilcooierto whichthedetail of Figs. 1-3 relates;

Fig. ,5 is a plan section of different embodiment of secondarywails;

Fig. 6 is a section inelevation alongthe-line of Fig;-

. Fig. 7 is--a:sectionaiongthe lineVII-VH of Fig.5;

Fig. 8 isa longitudinal section through an annular tube-heat exchangerpassage;

Fig. 9 is a cross-section at the-end of the tube shown in Fig. 8.

Fig. 10 is a part isometric view of an assembled oil-cooler-to which thedetail of .Figs- 5-7.relates.

In the embodiment shown in Figs. 1 to 4 inclusive, I denotes heatexchanger passages of elongated cross-sectional shape, with surfaces ofprimary heat transferring wall parallel to one another over the width ofthe passage. These passages, as seen on Fig. 4, are arranged between theentry header tank 6 (shown broken open) and the exit header tank 1below, the oil as indicated by black arrows 8, flowing through thepassage I from the top to the bottom header. Between these oil-passages,passages for air in cross-flow are formed, the air flow being indicatedby doubleheaded arrows.

As. seen with reference to Figs. 1 to 3, and also with reference to Fig.4, secondary walls 2 are mounted. in series, which series respectivelyextendfrom the entry to the exit cross-section of the passage. Eachsecondary wall 2 extends along the passage, has only a fraction of thelength of the primary heat transferring wall along the passage, and thesecondary walls of each series are arranged in step-wise staggeredprogression, with a gap 4 in the direction of the passage between thetrailing edge of one and the leading edge of the following secondarywall, the said gaps extending through all series of secondary walls, andwith each secondary wall of each series being parallel to, but out ofalignment along the flow-direction of the oil (indicated' by arrows 3 onFigs. 1 to 3) with every other secondary wall in the passage. Each ofthe secondary walls is arranged normally to the parallel facing surfacesof primary heat transferring wall in each passage, and connects to thesesurfaces at two opposite edges which extend along the passage, thepositions of contact forming an.

evenly distributive pattern over thesurfaces of primary heattransferring wall.

A seen with reference to Fig. 4, similar secondary walls 9 in thearrangement according to the-invention as described above for theoil-passages, are-provided in the air-passages.

The details shown with reference to Figs. 5 to '7, apply to anoil-cooler arrangement of tanks 6 and I with passags I as shown in Fig.10, and illustrate a different embodiment of secondary walls accordingto the invention inside the oil-passages l of elongated cross-sectionalshape. In contradistinction to. the embodiment of Figs. 1-4, secondarywalls I, each extending along the passage and of short lengthcomparedtherewith, and arranged in series l3, l4, l5 (indicated on Fig. 10) andI6 (indicated on Fig. 5) which series extend from the entry to exit ofthe passages, are each arranged parallel to the parallel side-parts la.and lb of primary heat transferring wall in a passage. In other words,secondary walls It extend longitudinally of the cross-sectionv of thepassage l. instead of transversely, s in the first embodiment. Thesecondary walls of alternate series It and I6 are respectively connectedto the side-part of primary heat transferring wall by carrying parts llbwhich extend normally oi the side-part lb, while the secondary walls ll!of the series l3 and I5 are connected to the side-part la 01' theprimary wall by the carrying parts lla,,as shown on Figs. 5, 6, 7 and10. Two opposite edges of each secondary wall ID are thus connected toprimary heat transferring wall, the contacts with the primary wallforming an evenly distributive pattern thereon. Connections l2 (seeFigs. 5 and 10) connect each series of carrying parts Ila and Ill; sothat the series of secondary walls with their carrying parts foreachpassageform one unit; to be inserted into the passage on assembly.The secondary walls of each series are staggered in step-wiseprogression, with gaps 4 along the passage between the trailing edge ofone and the leading edge of the following secondary wall, and with everysecondary wall of each series parallel to, but out of alignment alongsaid passage with every other secondary wall in the passage. The saidgaps A extend at the same level through all series of secondary walls,since the connection walls 12 are interrupted by these gaps.

Figs. 8 and 9 illustrate secondary walls according to this invention ina passage 22 of annular shape formed between two concentric tubes 20 and2| as primary heat transferring walls. Such passages might be visualizedas components of a heat exchanger in place of passages I on Fig. 4, onwhich a number of passages 22, suitably arranged, would replace onepassage l, the outer tubes fitting into the nearer surfaces of the tanks6 and l, and the inner tubes 2! into the outer surfaces of these tanks,so that oil would flow through the annular spaces and air past theoutside of tubes 22, while air or another cooling fluid might flowthrough the inside tubes 2!.

The secondary heat transferring walls 23, which each extend along thepassage and have only a fraction of the length of this passage, arearranged in four series. Each series of secondary walls extends thewhole length of the passage,

and the secondary wall 23 in it are staggered angularly in step-wiseprogression to cover respectively one quadrant of the annular space. Asin the embodiments described before, there are gaps 24 between thetrailing edge of each secondary wall and the leading edge of thefollowing secondary wall, and each secondary wall is out of alignmentwith every other secondary wall, and furthermore connects at twoopposite edges, which extend along the passage, to the primary heattransferring walls formed by tubes 20 and 2 I, these edges forming anevenly distributive pattern over the surfaces of the primary heattransferring walls.

It is seen with reference to all three of the embodiments described,that one sheet of metal, instead of forming one or a number of secondarywalls which extend from the beginning to the end of the passage, andhaving contact only with two layers of the fluid flowing in the passage(oil in this instance) forms according to this invention a series ofsecondary walls which are each of short length, are staggered instep-wise progression and have gaps along the passage between them,which come into contact with different layers of the flow of the fluid,and provide a considerable saving of metal if only due to the gaps andthe fact that each series covers a considerable portion of thecross-section of the passage, as seen with reference to the variousviews.

In operation then, each layer of fluid in the cross-sectional area offlow, irrespective of its thickness for effective heat exchange, comesinto contact with a secondary wall, and remains in contact therewithonly for such time (depend ing on speed of flow and length of secondarywall) a is required for it to take its required part in the heattransfer. Thereupon an adjacent pair of flow-layers reaches a followingsecondary wall of the series, so that all secondary walls of the seriesdisposed along the length of the pa,- sage transfer h at to the primaryheat transferring walls from layers of fluid which have not yet takendirect part in the heat transfer and thus still are near to theirentering temperature.

Therefore the temperature of the primary heat transferring wall, andthereby the temperature difference to the other fluid taking part in theheat exchange (air in this instance) is maintained along its length.This factor raises the rate of heat transfer per unit area of wallconsiderably compared with conventional constructions.

Furthermore, on secondary walls of any considerable length, stagnantboundary layers of the fluid, particularly if a liquid, are formed whichhave severe heat insulating effects.

In contradistinction to this, the effects of the gaps between thetrailing edge of one and leading edges of the following secondary wallare as follows:

(1) Due to the gaps, impact occurs on the fluid meeting the leading edgeof a secondary wall, producing a pressure, and suction occurs on thefluid leaving the trailing edge, producing a depression, both thesepressure changes being due to sudden changes in velocity at these edges.Since the secondary walls are purposely of short lengggiis impact andsuction will suflice to remo e stagnant layers from the secondary Walls,or at least to considerably reduce the thickness of such layers, thusgreatly enhancing heat transfer between the fluid and the secondarywalls.

(2) Due to the gaps extending right across the flow cross-section of apassage, the vortices due to such impact and suction can spread rightacross the flow cross-section, and produce intermixing of the fluidparticles of a kind which assists heat transfer between fluid andsecondary and primary heat transferring walls.

(3) A most important effect of the gaps consists in separating from oneanother the connections of the secondary walls with the primary heattransferring wall. This is important because the heat flowing from asource on a secondary wall, say, to a second fluid outside the primaryheat transferring wall flows into this second fluid not merely throughthe small contact-area between primary and secondary wall, but throughwider area of effectiveness surrounding this contact-area, since metalconducts heat o much better. If connections of secondary walls toprimary walls would not be separated, such areas of effectiveness of thevarious secondary walls would overlap on. the primary walls, which wouldbe wasteful since such overlapping portions of areas of effectivenesshave no more heat transfer than non-overlapping areas of effectiveness,and accordingly any part of the secondary wall causing such overlap issuperfluous. The gaps, which together with the step-wise staggering 01short secondary walls provide for separation of such areas ofeffectiveness and for even distribution of these over a surface ofprimary heat transferring wall, thus contribute in this way to improvedheat transfer over the whole area of primary heat transferring wall.

Due to the efiects of the provisions of this in-- vention, the width ofpassages for the fluid could be increased as compared to conventionalconstructions, thus considerably reducing the pressure loss of thefluid. This is still urther reduced by the fact that in anyflow-cross-section there are only few secondary walls, and none at allin the gaps, which factors in combination reduce the ratio of wettedcircumference to flow-cross-section which strongly influences pressureloss. With wider passages, fewer passages could handle the sameperformance, which further to the sav- I ingofmetaliomtheactualsecondary surfaeesand the saving dueto improved rate of heat; transfer,adds up to a considerable saving of metal.

The arrangement of secondary wall perpendfcularly' to the primary heattransferring-wall,

in Figs. 1-4 and 8' and 9 has the'following: advantage over thearrangement of secondary walls parallel to the primary heat transferringwalls: Each secondary wall is directly connected to both primary heattransferring walls, and comes into contact both with fluid near theprimary walls and centrally in the flow, which latter still have extremetemperatures. Hence between these and the primary wall the secondarywalls conduct heat with the greatest temperature difference along theshortest paths, and, simultaneously to'two primary walls.

Secondary walls according to the present invention may be suitablycorrugated, or provided with one corrugation each-the said corrugationrunning along the passage-to allow for thermal expansion or contraction.It will be understood further that secondary walls according to thisinvention may be plane or curved, or that there may be curved and planesecondary walls inthe same series.

Iclaim:

l; A heat exchanger comprising'at least one primary heat transferringwall forming, at'least in part, a channel'for a heat exchange fluidflowing therealong, and a series of secondary heat transferring walls,of which each extends along said channel and has a fraction of thelength of said primary heat transferring wall along said channel, thesaid secondary walls of said series being in a staggered arrangementrelative to one another with, for at least two consecutive secondarywalls of said series, a gap along said channel between the trailing edgeof the one and the leading edge of the consecutive secondary wall, andwith at least said two secondary walls of said series out of alignmentalong said channel with any secondary wall in said channel, and at leastone of said two secondary walls having two edges connected to a primaryheat transferring wall, the said edges extending along said channel.

2. A heat exchanger comprising at least one primary heat transferringwall forming, at least in part, a channel for a heat exchange fluidflowing'therealong, and a plurality of series of secondary heattransferring walls of which each secondary wall extends along saidchannel and has a fraction of the length of said primary heattransferring wall along said channel, the said secondary walls of eachseries being in a staggered arrangement relative to one another with,for at least two consecutive secondary walls of each series, a gap alongsaid channel between the trailing edge of the one and the leading edgeof the consecutive secondary wall, said gap extending across the widthof said primary heat transferring wall through all said series ofsecondary walls, and with at least said two secondary walls of eachseries out of alignment along said channel with any secondary wall insaid channel, and at least one of said two secondary walls ofeach serieshaving two edges connected to said primary heat transferring wall, thesaid edges extending along said channel.

3. A, heat exchanger comprising at least one primary heat transferringwall forming a substantially straight passage of elongatedcrosssectional shape with at least two opposing surfaces of. primaryheat transferring wall substantiallyparallel to one another, foraheat-exchange fluid flowing therealong, and a plurality of "series ofsecondary heat transferring walls, of which each secondary wall extendsalong'said passage and has a fraction of the length of said primary heattransferring wall along said passage, the said secondary walls of eachseries being in a staggered arrangement relative to one another with,for at least two consecutiv secondary walls of each series, a gap alongsaid passage between the trailing edge of the one and the leading edgeof the consecutive secondary wall, said gap extending across the widthof said primary heat transferring wall through all said series ofsecondary walls, and with at least said two secondary walls of eachseries out of alignment along said passage with any secondary wall insaid passage, and at least one of said two secondary walls of eachseries having two edges connected to said primary heat transferringwall, the said edges extending along said passage.

4. A heat exchanger as claimed in claim 3 in which each secondary wallof each series has two opposite edges in contact with said two opposingparallel surfaces of primary heat transferring wall, the said edgescontacting said primary wall being spaced from one another and beingarranged in an evenly distributive pattern on said surfaces of primaryheat transferring wall.

5. A heat exchanger comprising at least one primary heat transferringwall forming a substantially straight passage of elongatedcrosssectional shape with at least two opposing surfaces of primary heattransferring wall substantially parallel to one another, for aheatexchange fiuid flowing therethrough, and a plurality of series ofsecondary heat transferring walls, of which each secondary wall extendsalong said passage and normal to said opposing surfaces of primary heattransferring wall, and has a fraction of the lmgth of said primary heattransferring wall along the length of said passage, the said secondarywalls of each series being arranged in step-wise staggered progressionrelative to one another, with a gap along said passage between thetrailing edge of each secondary wall and the leading edge of eachconsecutive secondary wall, said gaps extending across the width of saidprimary heat transferring wall through all said series of secondarywalls, and with each secondary wall of each series parallel to and outof alignment along said passage with all other secondary walls, and eachsecondary wall of each series having two opposite edges connected withsaid opposite and parallel surfaces of primary heat transferring wall,the said edges extending along said passage, and having an evenlydistributive arrangement on said primary wall surfaces.

6. A heat exchanger comprising two primary heat transferring walls eachforming a passage of similar cross-sectional shape, one of said passagesbeing arranged within and coextensive with the other one to form apassage of substantially annular cross-sectional shape between said twoprimary heat transferring walls, for a heat exchange fiuid flowingtherethrough, and a plurality of series of secondary heat transferringwalls of which each secondary wall extends along said annular passage,ha a fraction of the length of said primary heat transferring wallsalong said passage, and extends substantially normally to the surfacesof said primary heat transferring walls, the said secondary walls ofeach series being staggered in spirally step-wise progression relativeto one another, with a gap along said passage between the trailing edgeof each sec ondary wall and the leading edge of the consecutivesecondary wall, the said gaps extending respectively through all saidseries of secondary walls across the whole cross-section of said annularpassage, and with each secondary Wall of series rotationally out ofalignment with all other secondary walls, and each secondary wall ofeach serie having two opposite edges respectively connected with saidtwo primary heat transferring walls, the said edges extending along saidpassage, and being evenly distributed over said primary walls.

7. A heat exchanger comprising at least one primary heat transferringwall forming a substantially straight passage of elongatedcrosssectional shape for a heat exchange fluid flowing therethrough, thesaid primary heat transferring wall comprising two elongated side-partssubstantially parallel to one another and forming the elongatedside-walls of said passage; the said heat exchanger further comprising aplurality of series of secondary heat transferring walls, each secondaryWall of each series extending along said passage and substantiallyparallel to said elongated side-parts of said primary heat 10transferring wall, and having a fraction of the length of said primaryheat transferring wall along the length of said passage, the saidsecondary walls of each series being staggered in step-Wise progressionrelative to one another, with a gap along said passage between thetrailing edge of each secondary wall and the leading edge of eachconsecutive secondary wall, and; with each secondary wall of each seriessubstantially parallel to, but out of alignment along said.f.pas sage,with every other secondary Wall in said passage, and each secondary wallof each series having two edges connected to one of said sideparts ofsaid primary heat transferring wall; the said edges extending along saidpassage.

MEYER FRENKEL.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date;

2,133,502 Emmons Oct. 18, 1938 2,148,204 Kemp Feb. 21, 1939 2,149,696Holmes Mar. 7, 1939 2,173,844 Houdry Sept. 26, 1939 2,480,706 BrinenAug. 30, 1949

